Speed up Cycles Render: Only calculate whats visible to the camera
It's not really limiting the render to what's visible to the camera, but you can limit some effects to camera, such as bump maps.

Mixing your bump BW factor with a 50% grey color with light path's "is camera ray" to make your bump map only visible to the camera. Add (math node, clamp checked) "is camera ray" with "is glossy ray" to have your bump visible in glossy reflections too.